As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the next generations, since they can be easily small-sized while requiring a greatly reduced voltage amplitude.
Organic EL devices generally have a basic structure of positive electrode/light-emitting layer/negative electrode, in which a transparent positive electrode is positioned over a substrate such as a glass sheet. In those, the light emission is taken out through the side of the substrate.
For the following reasons (a) to (c), transparent negative electrodes are disclosed as the recent approach in the art. The light is taken out through the side of the negative electrode for the organic devices with the transparent negative electrodes.
(a) In the EL devices of that type, where the positive electrode is also transparent, one can obtain transparent EL devices.
(b) Any desired color can be employed as the background color for the transparent EL devices. Therefore, the EL devices could be colorful even while they do not emit light, and could be decorative ones. Where the background color for the EL devices is black, the contrast for those devices is improved.
(c) A color filter or a color-converting layer, if used, could be disposed over the transparent EL devices. Therefore the EL devices can be fabricated without considering the heat resistance of those filter and layer. For example, one advantage is that when the positive electrode for the EL devices is formed, the substrate temperature can be increased to thereby lower the resistance value of the positive electrode formed.
On the other hand, the recent tendency in the art is toward high-resolution and large-sized displays comprising organic EL devices, and high-resolution display devices require fine pixels of not larger than several hundreds .mu.m.sup.2 in size. In such high-resolution display devices, the scanning electrode lines and the signal electrode lines shall become very narrow, resulting in that they shall have higher resistance. In those, however, the scanning electrode lines and the signal electrode lines with such high resistance are problematic in that the voltage level in the devices is lowered and that the devices being driven produce delayed responses. Specifically, the voltage drops make the luminance of the devices inhomogeneous, while the delayed responses make it difficult to image moving pictures on the display. For these reasons, the high-resolution display devices with such disadvantages are problematic in that their displays are limited.
In those display devices, the scanning electrode lines and the signal electrode lines are connected with the lower electrodes and the upper electrodes constituting the organic EL devices. Therefore, the positive electrodes and the negative electrodes for those lower electrodes and the upper electrodes are required to have lower resistance values.
Transparent negative electrodes, if used in organic EL devices, have various advantages such as those mentioned hereinabove. Therefore, various approaches are being tried in the art to the production of organic EL devices with transparent negative electrodes.
Japanese Patent Application Laid-Open No. 185984/1996 discloses a transparent EL device, which comprises a first electrode layer of a transparent conductive layer, and a second electrode layer composed of an ultra-thin, electron injection metal layer and a transparent conductive layer formed thereon. However, this does not disclose at all any technical idea of lowering the resistance of those electrode layers. In this, used are ITO (indium tin oxide) and SnO.sub.2 as the substances constituting the transparent conductive layers. However, it is impossible to make these substances non-crystalline to such a degree that they give no peak in their X-ray diffraction patterns, and therefore the substances used in the disclosed technique are naturally crystalline ones. Accordingly, where a layer of any of these substances is positioned onto a substrate via an organic layer while the substrate temperature is kept at about room temperature to 100.degree. C. or so in order to prevent the organic layer from being damaged, the transparent conductive layer formed shall have a high specific resistance value. For example, for ITO, its layer shall have a specific resistance of not smaller than 1.times.10.sup.-3 .OMEGA..multidot.cm or so. In the organic EL devices having such a transparent conductive layer with such a high specific resistance, the voltage is lowered at the wired lines of the transparent conductive layer, resulting in that the light emission is inhomogeneous. Therefore, some improvements are desired for lowering the specific resistance of the conductive layer in those EL devices. In addition, since ITO and SnO.sub.2 are naturally crystalline substances, water and oxygen easily penetrate thereinto through their grain boundaries. Therefore, the electron injection metal layers to be laminated adjacent to the conductive layers of those substances, ITO and SnO.sub.2, are easily deteriorated, thereby producing light emission defects or even failing in light emission. Thus, the transparent EL device disclosed does not have satisfactory durability, and further improvements therein are desired. If the transparent organic EL device of that type, in which the negative electrode is made of only one crystalline, transparent conductive layer, is used in high-resolution display devices, the voltage will be lowered at the wired lines of the transparent conductive layer to thereby make the light emission inhomogeneous. Therefore, the use of the EL device itself is limited. In addition, since ITO and SnO.sub.2 are naturally crystalline substances, water and oxygen easily penetrate thereinto through their grain boundaries. Therefore, the electron injection metal layers to be laminated adjacent to the conductive layers of those substances, ITO and SnO.sub.2, are easily deteriorated, thereby often producing light emission defects or even failing in light emission. Accordingly, it is desired to further improve the durability of the organic EL device.
For the crystalline transparent conductive layer used in the disclosed technique, it is difficult to employ a so-called taper etching process for forming an etched pattern of the layer having a trapezoidal cross-sectional profile in producing organic EL display devices having an XY matrix structure. Therefore, using the crystalline transparent conductive layer, it is often difficult to produce high-resolution display devices.